This application addresses latent chronic infection caused by the protozoan parasite Toxoplasma gondii. In the host, Toxoplasma converts from a proliferative stage (tachyzoite) that causes acute infection to a latent cyst (bradyzoite) stage tht establishes a lifelong chronic infection. Chronic toxoplasmosis is of great clinical importance because the latent infection can reactivate into life-threatening acute toxoplasmosis, most notably in AIDS and heart transplant patients. This application addresses the significant need to develop therapies that can prevent latent cysts from reactivating infection. Our previous findings established a role for eIF2 (eukaryotic initiation factor-2) phosphorylation in microbial latency, prompting us to test if inhibitors of eIF2 dephosphorylation could interfere with reactivation of latent Toxoplasma infection in vitro. We found that one such drug, guanabenz (GA), is a potent anti-parasitic agent that also inhibits the reactivation of bradyzoite cysts in vitro. Importantly,GA is a well-tolerated FDA-approved drug that crosses the blood-brain barrier, properties that make it an outstanding candidate for rapid advancement as an innovative new treatment for chronic toxoplasmosis. In the R21 phase of this application, we will test the utility of GA in a well-characterized mouse model of chronic infection, and conduct a screen for GA-resistant parasites in order to assess the likelihood and mechanism of drug resistance. These mutants will facilitate studies to define how this drug inhibits parasite proliferation and the reactivatio of infection. Milestones for advancement to the R33 phase include either a demonstration of GA activity against chronic Toxoplasma in vivo or the generation of GA-resistant mutants. If one or both of these milestones are achieved, the R33 phase of this study will elucidate the molecular mechanism underlying the ability of GA to control parasite replication and latent infection using complementary biochemical and genetic approaches.